1. Field of the Invention
The present invention relates to a thermal activation device for heat-sensitive self-adhesive sheet and a printer assembly employing the thermal activation device, the heat-sensitive self-adhesive sheet having a heat-sensitive adhesive layer which is formed on one side of a sheet-like substrate of the sheet and which is normally non-adhesive but develops adhesiveness when heated. Particularly, the invention relates to a technique for effectively removing heat-sensitive adhesive mass adhered to a platen roller.
2. Description of the Related Art
In recent years, a thermally active sheet (a print medium, such as a heat-sensitive self-adhesive sheet, which has a coat layer of a thermally active component formed on top surface thereof) has been known as a kind of sheet affixed to products. The thermally active sheets have found a wide range of applications such as POS sheets affixed to food products, affixing sheets used in physical distribution/delivery, sheets affixed to medical products, baggage tugs, indication sheets affixed to bottles or cans and the like.
The heat-sensitive self-adhesive sheet R includes a sheet-like sheet substrate (such as a base paper); a heat-sensitive adhesive layer formed on a back side of the substrate and being normally non-adhesive but developing adhesiveness when heated; and a printable surface formed on a front side of the substrate.
Specifically, as shown in FIG. 7, a thermal coat layer 501 as a heat-sensitive color developing layer defining a printable surface is formed on one side of a base paper 500 (front side as seen in FIG. 7) as the sheet substrate, and a colored print layer 502 printed with characters or a pattern, such as a price frame, unit and the like, is formed on the thermal coat layer. On the other side of the base paper (back side as seen in FIG. 7), a heat-sensitive adhesive layer K is formed by applying a heat-sensitive adhesive based on a thermoplastic resin, a solid plasticizer and the like.
The heat-sensitive adhesive includes a thermoplastic resin, a solid plasticizer and the like as the major components thereof, and has a nature that the heat-sensitive adhesive is non-adhesive at normal temperatures but is activated to develop the adhesiveness when heated by the thermal activation device. Normally, activation temperatures are in the range of 50 to 150° C., in which range the solid plasticizer in the heat-sensitive adhesive is molten to impart the adhesiveness to the thermoplastic resin. The molten solid plasticizer is gradually crystallized via a supercooled phase so that the adhesiveness is maintained for a given period of time. While the heat-sensitive adhesive exhibits the adhesiveness, the sheet is affixed to a support object such as a glass bottle or the like.
The heat-sensitive self-adhesive sheet R is subjected to a thermal printer assembly with a thermal head for printing a desired character(s) or image on the printable surface thereof and thereafter, subjected to the thermal activation device for activation of the heat-sensitive adhesive layer K thereof.
On the other hand, a printer assembly is now under development, which incorporates therein the thermal activation device for sequentially conducting thermal printing on the heat-sensitive self-adhesive sheet and activation of the heat-sensitive adhesive layer thereof.
Such a printer assembly has an arrangement as shown in FIG. 6, for example.
Referring to FIG. 6, a reference sign P1 represents a thermal printer unit, a sign C1 representing a cutter unit, a sign A2 representing a thermal activation unit, a sign R representing a heat-sensitive self-adhesive sheet wound into a roll.
The thermal printer unit P1 includes a printing thermal head 100, a platen roller 101 pressed against the printing thermal head 100, and an unillustrated drive system (including an electric motor, and gear array, for example) for rotating the platen roller 101.
As seen in FIG. 6, the platen roller 101 is rotated in a direction D1 (clockwise) thereby paying out the heat-sensitive self-adhesive sheet R, which, in turn, is subjected to thermal printing and then discharged in a direction D2 (rightward). The platen roller 101 further includes unillustrated pressure means (such as a helical spring or plate spring), a resilient force of which acts to bias the platen roller 101 surface against. the thermal head 100.
The printing thermal head 100 and platen roller 101 are operated based on a print signal from an unillustrated print control unit, thereby accomplishing desired printing on the thermal coat layer 501 of the heat-sensitive self-adhesive sheet R.
The cutter unit Cl serves to cut the heat-sensitive self-adhesive sheet R, thermally printed by the thermal printer unit P1, in a proper length. The cutter unit includes a movable blade 200 operated by a drive source (not shown) such as an electric motor, and a fixed blade 201. The movable blade 200 is operated at a predetermined timing under control of the unillustrated control unit.
The thermal activation unit A2 includes an insertion roller 300 and a discharge roller 301 rotated by, for example, an unillustrated drive source for inserting and discharging the cut heat-sensitive self-adhesive sheet R; a thermally-activating thermal head 400 interposed between the insertion roller 300 and the discharge roller 301; and a platen roller 401 pressed against the thermally-activating thermal head 400. The platen roller 401 includes an unillustrated drive system (an electric motor and gear array, for example), which rotates the platen roller 401 in a direction D4 (a counter-clockwise direction as seen in FIG. 6) so that the heat-sensitive self-adhesive sheet R is conveyed in a direction D6 (a rightward direction as seen in FIG. 6) by the insertion roller 300 and discharge roller 301 rotated in respective directions D3 and D5. On the other hand, the platen roller 401 includes unillustrated pressure means (such as a helical spring or plate spring), a resilient force of which acts to bias the platen roller 401 surface against the thermally-activating thermal head 400.
In FIG. 6, a reference sign S represents a discharge detection sensor for detecting the discharge of a heat-sensitive self-adhesive sheet R. The printing, conveyance and thermal activation of the subsequent heat-sensitive self-adhesive sheet R are performed in response to the discharge detection sensor S detecting the discharged heat-sensitive self-adhesive sheet R.
The thermally-activating thermal head 400 and the platen roller 401 are operated at a predetermined timing under control of the unillustrated control unit, while the heat-sensitive adhesive layer K of the heat-sensitive self-adhesive sheet R is activated by heat generated by energizing the thermally-activating thermal head 400, thereby developing an adhesive force.
After the adhesive force of the heat-sensitive self-adhesive sheet R is developed by the thermal activation unit A2 thus arranged, an indication label, price label or advertisement label may be affixed to glass bottles containing liquors or medical agents or to plastic containers. This negates the need for a separation sheet (liner) provided at the adhesive label sheet commonly used in the art, providing a merit of cost reduction. In addition, the invention provides further merits in terms of resource savings and environmental problems because the separation sheets producing wastes after use are not required.
However, the conventional thermal activation unit A2 for heat-sensitive self-adhesive sheet R encounters a problem that the heat-sensitive adhesive is adhered to conveyance means (particularly, the platen roller 401) for the heat-sensitive self-adhesive sheet R.
Specifically, when the heat-sensitive self-adhesive sheet R cut in a predetermined length by the cutter unit C2 is thermally activated at the heat-sensitive adhesive layer K thereof by means of a heat generating element H of the thermally-activating thermal head 400 and then released from the platen roller 401, a part of the heat-sensitive adhesive of the heat-sensitive adhesive layer K, softened (liquefied) by heating, is squeezed out between the platen roller 401 and the thermally-activating thermal head 400, thus separated from the base paper 500 of the heat-sensitive adhesive sheet R, as shown in FIG. 8A.
Furthermore, a separated heat-sensitive adhesive mass G1, as shown in FIG. 8A, has the adhesive force developed by the activation and hence, adheres to a peripheral surface of the platen roller 401 temporarily idling after the discharge of the heat-sensitive self-adhesive sheet R, as shown in FIG. 8B.
While the platen roller 401 is subjected to the state shown in FIGS. 8A and 8B in several cycles, the platen roller 401 sustains the adherence of multiple heat-sensitive adhesive masses G1 to its peripheral surface, as shown in FIG. 8C.
Furthermore, the heat-sensitive adhesive masses G1 on the periphery of the platen roller 401 are molten by repeated heating by the thermally-activating thermal head 400, thus exhibiting a strong adhesive force. Accordingly, some of the adhesive masses adhere to a surface of the subsequent heat-sensitive self-adhesive sheet R, contaminating a printable surface thereof.
In addition, there exists a problem that the peripheral surface of the platen roller 401 is deteriorated in smoothness due to the adherence of multiple heat-sensitive adhesive masses G1 and hence, the subsequent heat-sensitive adhesive layer K of the heat-sensitive self-adhesive sheet R cannot be uniformly heated, thus failing to exhibit a sufficient adhesive force.
In order to eliminate such problems, a user needs to regularly remove the masses adhered to the periphery of the platen roller with a cleaning solvent or exchange the platen rollers. This is cumbersome and also increases maintenance costs.